1. Field of the Invention
This invention relates to chemical vapor deposition reactions, integrated circuit manufacturing and, more particularly, to methods for depositing titanium metal layers on in-process integrated circuits.
2. Description of the Related Art
Deposited titanium metal layers are being used with increasing frequency in integrated circuits. One important application involves the formation of contact structures within a dielectric layer. The processing of wafers for the manufacture of integrated circuits commonly requires that contact openings be etched through an insulative layer down to implant or diffusion regions in a semiconductor layer to which electrical contact is to be made. Titanium metal is then deposited over a wafer so that the surface of each exposed implant/diffusion region is coated. The titanium metal is eventually converted to titanium silicide. A silicide is a binary compound formed by the reaction of silicon with the metal at elevated temperatures. The titanium silicide layer serves as an excellent conductive interface at the surface of the implant/diffusion region. A titanium nitride barrier layer is then deposited, coating the walls and floor of the contact opening. The contact plugs are formed by depositing a tungsten or polysilicon layer via chemical vapor deposition. In the case of tungsten, the titanium nitride layer provides greatly improved adhesion between the walls of the opening and the tungsten metal. In the case of the polysilicon, the titanium nitride layer acts as a barrier against dopant diffusion from the polysilicon layer into the diffusion region.
Deposited titanium metal layers are also used as an underlayment for aluminum alloy layers deposited on interlevel dielectric layers. The titanium and aluminum alloy layer stack is etched to form interconnect lines within the integrated circuit. The titanium metal layer not only provides increased resistance to electromigration of aluminum atoms, but also provides improved adhesion of the aluminum alloy layer to the dielectric layer as compared with an aluminum alloy layer without the titanium underlayment.
Two principal techniques are presently available for creating thin titanium films: deposition via reactive sputtering of a titanium target and chemical vapor deposition. When topography is present, reactive sputtering results in titanium films having poor step coverage. Although collimated sputtering improves the coverage on trench floors, it does not help coverage on vertical surfaces. In fact, as trench aspect ratios (the ratio of depth to width) exceed 4 or 5 to 1, the deposition rate at the bottom of the trench is minimal because of the buildup of deposited metal at the mouth of the trench. As the mouth of the trench narrows during the deposition process, the corners of the trench floor receive increasingly less deposited material. Because of the step-coverage problem, sputter-deposited films are limited primarily to underlayment layers on relatively planar surfaces. Another problem related to collimated sputtering is that the collimator grid dramatically slows the deposition rate and must be cleaned frequently.
Chemical vapor deposition processes have an important advantage over sputter deposition techniques in that the deposited layers have much higher conformality (i.e., uniform thickness on both horizontal and vertical surfaces), layers of any thickness may be deposited, and the deposition rate does not slow with time (as with collimated sputtering). This is especially advantageous in modern ultra-large-scale-integration (ULSI) circuits, where minimum feature widths may be smaller than 0.3 .mu.m and trenches and contact openings may have width to depth aspect ratios of 1:5 or more. In U.S. Pat. No. 5,173,327, a chemical vapor deposition process for titanium is disclosed. Titanium tetrachloride and hydrogen gas are admitted to a chemical vapor deposition chamber in which a substrate (i.e., semiconductor wafer) has been heated to about 400.degree. C. Titanium tetrachloride molecules are adsorbed on the substrate surface and react with hydrogen with the following chemical equation: TiCl.sub.4 +2H.sub.2 =Ti+4HCL. The deposition rate of this reaction can be enhanced by striking a radio-frequency plasma in the deposition chamber. Because the diatomic hydrogen molecule is relatively difficult to ionize, the flow rate of hydrogen gas into the deposition chamber must be considerably greater than that for titanium tetrachloride. The low ratio of titanium tetrachloride molecules to hydrogen molecules is not conducive to high deposition rates. In addition, as the aspect ratio of trenches and contact openings increases, step-coverage worsens due to the limited amount of titanium tetrachloride that is adsorbed toward the bottom of the trenches and contact openings. Although the aforementioned titanium deposition process is satisfactory for many applications, the present invention aims at providing a chemical vapor deposition process for titanium having increased conformality and more rapid deposition rates.